Conventionally, boilers using solid fuels are widely known. In particular, boilers using wood pellets as a solid fuel have recently attracted attention from the standpoint of environmental load reduction since wood pellets are a carbon-neutral fuel.
Patent Literature 1 discloses a pellet boiler 100 shown in FIG. 8. Pellets stored in a hopper 114 are supplied to a hearth 111 by a screw feeder 115. In the hearth 111, the pellets are combusted and a combustion gas is generated. The combustion gas flows through a coiled or labyrinthine path in a heat exchanger 113, and is discharged outside the boiler 100 through a discharge tube 117. The heat exchanger 113 has formed therein a labyrinthine path through which a heat transfer medium used for heating or the like flows. In the heat exchanger 113, the heat transfer medium is heated by the combustion gas.